In "To Kill a Mockingbird", what does Jem do to make Scout angry?

Don't you mean when he tells on Dill when the Finch children find him hiding under the bed?

Jem knows it will probably get Dill into temporary trouble to be "found out" (Dill was unhappy and bored at home and had run away to Maycomb), but he is old enough at this point to know that the adults need to be informed of Dill's whereabouts. Dill couldn't just camp out at their house while he was listed as missing. Also, his mother was sick with worry.

In a way, this was a distress signal from Dill that his mother and stepfather had been neglecting him ever since his mother had married. Dill, an illigitimate child (sent off to his aunt's place during the summertime and enjoying it), also needed a place in their hearts to "belong."